This invention relates to the use of blocked urethane prepolymers as wet strength agent to be added to the stock to obtain paper with increased strength in the wet state. It also relates to a process for making paper with increased wet strength including the addition of a blocked urethane prepolymer to the stock.
Paper comprises, in addition to cellulose fiber, a variety of additives such as fillers, strength agents and the like. Wet strength agents are an additive which gives paper having increased strength in the wet state. Examples of widely used wet strength agents include urea resins, melamine resins, dialdehyde starch, polyethyleneimine, epoxylated polyamide etc.
Recently, several attempts have been made to use polyurethanes and urethane prepolymers as wet strength agent. JP-A-06173196 discloses use of a urethane prepolymer containing tertiary amine or quaternary ammonium moieties without blocking the free isocyanate groups. The prepolymer is either added into the stock or applied to webs formed therefrom in the form of an aqueous dispersion or emulsion. GB 2068034A discloses use of a urethane prepolymer amine salt as wet strength agent. The prepolymer amine salt is produced by blocking a urethane prepolymer having free isocyanate groups with a ketoxime, and reacting the blocked urethane prepolymer with a polyfunctional amine to give amine containing-blocked urethane prepolymer followed by dissolving the reaction product in water in the form of an acid addition salt. This amine salt is either added to the stock or applied to a web formed therefrom. JP-A-05051896 discloses a wet strength agent of the type to be applied to a web or paper. The agent comprises a water-soluble urethane prepolymer blocked with sodium hydrogen sulfite or a water soluble urethane prepolymer blocked with ketoxime or other blocking agent which is solubilized by introducing a hydrophilic moiety using the reaction with dimethylolpropionic acid.
Urethane prepolymers containing free isocyanate groups are generally unstable in the presence of water. Blocked urethane prepolymers, on other hand, require heating for unblocking. For blocked urethane prepolymers to be added in the stock, it is imperative to be unblocked under conditions to be encounted in the dryer part of paper making machines (up to 130xc2x0 C. in several minutes).
A need exists, therefore, a water-soluble blocked urethane prepolymer which, when added in the stock as wet strength agent, is capable of unblocking under conditions to be normally encountered in the dryer part of paper making machines.
The present invention relates to the use of a water-soluble blocked urethane prepolymer as paper wet strength agent. According to the present invention, said blocked urethane prepolymer is blocked with a phenolic compound having an electron-attracting group at the ortho- or para-position. This blocking agent is capable of unblocking at a temperature lower than the unblocking temperature of oximes and, therefore, water-soluble urethane prepolymer blocked therewith may be used for producing paper with increased wet strength by adding in the stock without need for modifying conventional or existing paper making machines.
The present invention provides, therefore, a method for making paper with increased wet strength comprising:
adding a water-soluble, blocked urethane prepolymer having a hydrophilic group and a blocked isocyanate group blocked with a phenolic compound having an electron-attracting group at the ortho- or para-position to the stock, forming a web from the stock, and drying the web at a temperature up to 130xc2x0 C.
Suitable examples of said phenolic compounds include p-hydroxybenzoic acid, esters thereof, salicylic acid and esters thereof. They are used as preservative of foods and pharmaceutical preparations due to their safety to human beings. Accordingly, the presence thereof in paper is out of concern if they are produced by the unblocking reaction of the blocked prepolymer and remain in paper. Also, if they are contained in the effluent from paper mills, the ecological affects thereof are considered to be minimum.
As is well-known in the art, urethane prepolymer are produced by reacting a polyisocyanate with a polyether or polyester polyol at an NCO/OH equivalent ratio greater than 1. Examples of the starting polyether polyols, polyester polyols and polyisocyanates are all well-known in the polyurethane industry. Any of known materials may be used in the present invention.
Examples of suitable polyether polyols include those having a functionality of at least three and a molecular weight from 1,000 to 50,000. These polyether polyols may be prepared starting from a polyhydric alcohol such as glycerine, trimethylol propane, pentaerythritol and the like as an initiator by addition reaction with an alkylene oxide such as ethylene oxide or propylene oxide in the presence of a basic catalyst.
The use of polyester polyols as polyol component of the urethane prepolymer is effective to increase the wet strength of paper. Polyester polyols are generally produced by reacting an dihydric alcohol with a dicarboxyl acid or its acid hydride. Examples of dihydric alcohols include ethylene glycohol, 1,4-butanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol and other aliphatic glycols. Also included in usable dihydric alcohols are polyether polyols produced by the addition-polymerizing reaction of glycols or bifunctional phenols such as bisphenol A with ethylene oxide and/or propylene oxide and/or butylene oxide. Examples of dicarboxylic acids and acid anhydrides include adipic, terephthalic, isophalic or sebacic acid and maleic anhydride. Ring opening polymers of a lactone such as xcex5-caprolactone or a cyclic carbonate such as trimethylene-carbonate are included in the class of polyester polyols. Polyester polyols having a functionality greater than two may be produced by using a polyhydric alcohol having a functionality greater than two as a portion of the alcoholic component.
The starting polyisocyantes are preferably non-yellowing polyisocyanates, namely polyisocyanates not containing an aromatic ring. Examples thereof include aliphatic diisocyanates such as hexamethylenediisocyanate, alicyclic diisocyanates such as isophoronediisocyanate, dimers, trimers or adducts thereof with a polyhydric alcohol such as trimethylolpropane.
A first method for introducing a hydrophilic group into the blocked urethane prepolymer comprises the steps of reacting a portion of free isocyanate groups in the urethane prepolymer produced from a polymer polyol component and a polyisocyanate component with a hydrophilic group-introducing active hydrogen compound (hydrophilic group precursor), and then reacting said phenolic blocking agent to block the remaining free isocyanate groups.
The urethane prepolymer must have a free isocyanate content of at least 2% by weight and preferably from 5 to 10% by weight. A portion of the free isocyanate content is consumed by the reaction with the hydrophilic group-introducing active hydrogen compound. A number of such active hydrogen compounds are disclosed in JP-A-06017196, GB 2068034A and JP-A-05051896 cited above. Any of disclosed active hydrogen compounds may be used in the present invention. However, preference is made to an aliphatic active hydrogen compound having a tertiary amino group and an active hydrogen-containing group such as hydroxyl or primary amino group. Typical examples thereof are N,N-dimethyldiethanolamine, N,N-dimethylethylenediamine or their homologs. Dihydric tertiary aminoalcohols are preferable. The hydrophilic precursor compound is bound to the urethane prepolymer by the reaction between the active hydrogen-containing group thereof and a portion of the free isocyanate groups of the urethane prepolymer. The remainder of free isocyanate groups is then blocked with a blocking agent disclosed herein. The water-soluble blocked urethane prepolymer is prepared by quaternization of bound tertiary amino group with a quaternizing agent such as dimethyl sulfate. Alternatively, the bound tertiary amino group may be neutralized with a suitable acid to solubilize the blocked urethane in the form of an acid addition salt. Solubilization in the form of a quaternary ammonium salt is preferable. This is because it does not need for an acid for solubilization and, therefore, the stock may be prepared in neutral to basic regions.
The ratio of the hydrophilic group-introducing active compound to the urethane prepolymer must be sufficient to render the urethane prepolymer water-soluble but not sufficient to block all of free isocyanate groups in order that at least a portion of free isocyanate groups remain unreacted. This ratio naturally varies depending on the nature of the starting polyol and polyisocyanate components of the urethane prepolymer and also on the type of a particular hydrophilic group-introducing active compound employed. This ratio generally lies between 10% and 50% equivalents relative to the free isocyanate content of the urethane prepolymer.
The balance of free isocyanate content of the urethane prepolymer is blocked with a phenolic compound having an electron-attracting group at the ortho- or para-position. A phenolic compound having an amino group at the designated position is excluded from the blocking agent used in the present invention. This is because amino group irreversibly forms a urea linkage by the reaction with a free isocyanate group and the reaction dose not fall in the usual sense of xe2x80x9cblockingxe2x80x9d. Phenols having an electron-attracting group such as trifluoromethyl, nitro, cyano, acetyl, alkoxycarbonyl, carboxyl or alkylsulfonyl group at the designated position may be used. Examples of most preferable electron-attracting groups include free or esterified carboxyl group. Examples of specific compounds having such electron-attracting groups are p-hydroxybenzoic acid, esters thereof, salicylic acid and esters thereof. They are in use in foods, cosmetics or pharmaceutical preparations as a preservative or analgesic due to safety to human beings and the environment. Accordingly, safety concern is minimum if the preferred blocking agent remains in paper or is contained in the effluent from the paper mill.
A second method for introducing a hydrophilic group into the blocked urethane prepolymer includes to use a polyhydric alcohol having a tertiary amino group as a portion of the polyol component in the synthesis of the urethane prepolymer. Examples of suitable tertiary amino alcohols include N-methyldiethanolamine, N-ethyldiethanolamine and like N-alkyldialkanolamines. Triethanolamine could also be used. This method is especially suited where a polyester polyol is used as the polyol component in the synthesis of the urethane prepolymer. If the polyol component comprises a bifunctional tertiary amino alcohol and a bifunctional polyester polyol, then a urethane prepolymer having a functionality greater than two may be produced by incorporating into the reaction system a low molecular weight polyhydric alcohol having a functionality greater than two such as glycerine or trimethylolpropane. In this case, the NCO/OH equivalent ratio of the polyisocyanate component must be excess of 1 relative to the sum of OH groups possessed by the polyether polyol, polyester polyol, polyfunctional tertiary amino alcohol and the low molecular weight polyhydric alcohol having a functionality greater than two.
All of free isocyanate groups of the resulting urethane prepolymer having a hydrophilic group or a precursor group thereof are blocked with the blocking agent mentioned above with regard to the first method and then the precursor group (tertiary amino group) is quarternized with a quaternizing agent such as dimethyl sulfate or neutralized with a suitable acid as in the first method to give a water-soluble blocked urethane prepolymer having a hydrophilic group and a blocked isocyanate group.
The water-soluble blocked urethane prepolymer of the present invention is added in the stock together with other additives such as filler. The stock is then processed, as in the conventional paper making process, through the wire section, the press section and the dryer section to give paper having increased wet strength. The paper is dried by passing a series of cylinders which are heated usually to a maximum temperature up to 130xc2x0 C. . The water-soluble blocked urethane prepolymer of the present invention may be fully unblocked under the above heating conditions to regenerate reactive urethane prepolymer in situ which serves as binder to strengthen the binding of fibers. The water-soluble blocked urethane prepolymer could also find use as wet strength agent to be applied onto ready-made paper or webs as in the case of sizing agent.
The amount of the blocked urethane prepolymer of the present invention to be added to paper may vary depending on the desired strength property in the finished paper and generally ranges from 0.1% to 5% by weight as solids based on dry weight of fiber (pulp). This amount may be increased in excess of 5% by weight where higher wet strength is desirable.